An increasing number of active medical implants that require an electrical power source are being developed. Such devices include pacemakers, artificial hearts, drug pumps, neural stimulators, cochlear implants, retinal implants, and various other sensors, monitors and interfacing medical devices. Traditionally such devices have been powered by implanted batteries in the case of low-power devices (e.g., <1 mW), or via a magnetic induction link in the case of higher power devices (e.g., >1 mW). Magnetic induction links include two coils, one external and one implanted. When a current is driven through the external coil, a voltage is induced on the internal coil through magnetic induction that can be used to power the implanted device. Magnetic induction coils suffer from several major disadvantages, however. A main source of inefficiency is in ohmic heating of the transmit coil which can only be alleviated by increasing the diameter of the wire used in the coil which generally increases the size of the device. For example, the induction coils used to power cochlear implants with 30-80 mW typical power consumption are at least 50 mm in diameter. The large size of the external coil unit makes it hard to attach to the body without also implanting a permanent magnet. This improves the grip of the coil unit during charging; however, magnets can interfere with MRI scanning, an increasingly common diagnostic tool. Additionally, for the transmission of very large amounts of power (e.g., >1 W), the heating of the external coil can make the external unit hot, causing discomfort to the patient.
As an alternative to magnetic induction links, acoustic energy can be used to transmit power to the implanted device. This method uses an external send transducer to convert electrical energy into acoustic energy/waves and directs this energy through the patient's skin toward an implanted receive transducer that converts the acoustic energy back to electrical energy. The received electrical energy may be used to directly power a medical device or may be stored in a capacitor or a battery for later use. Several researchers have proposed devices that use acoustic energy to power implanted medical devices. To date, such devices have been limited by their low efficiency, their high sensitivity to alignment, and safety issues surrounding the heating of tissues. In recent years piezoelectric materials with a higher inherent electromechanical coupling coefficient have become available, notably the relaxor-PT materials. Such materials have made possible the achievement of high power transmission efficiencies (>20%) through several millimeters of tissue, but acoustic power transmission has still not supplanted inductive links. There are a number of reasons for this: acoustic links have historically required a high degree of angular and lateral alignment between the send and receive transducer; the receive transducer must be packaged in a hermetic, yet still acoustically transparent manner, which is difficult to achieve; and impedance matching to a load is often inefficient. Thus, there remains a need to produce more efficient, reliable, and easy-to-use acoustic-link power conversion systems for active implantable medical devices.